Fuel assemblies of nuclear reactors cooled with light water are generally formed by a bundle of fuel rods, i.e., tubes of a material having a low neutron absorption power, filled with pellets of fissile material. The bundle is usually assembled by means of elements disposed at each of the ends of the assembly. For the transverse maintenance of the fuel rods in the bundle, spacer-grids are generally used which are spaced apart along the length of the rod. Each of the spacer-grids defines an even network of passages adapted to receive the fuel rods or guide tubes which are substituted for certain fuel rods within the bundle. The guide tubes are generally connected to end elements or plates disposed at each of the ends of the assembly and in this way ensure the cohesion and the rigidity of the assembly. Furthermore, the guide tubes guide the absorbent rods which are interconnected to constitute clusters which may be shifted along the length of the assembly. These clusters or control bars ensure the control of the nuclear reactor by the displacement thereof in the direction of the height of the core of the reactor which is generaly constituted by assemblies placed side by side in the vertical direction.
Furthermore, in some types of reactor of recent design, called "spectrum variation reactors", certain guide tubes enable spectrum variation bars to be introduced in the core.
In the fuel assemblies of reactors prsently in service, in particular pressurized water nuclear reactors, the spacer-grids are generally constituted by small plates of thin zirconium alloy sheets assembled together by welding. Each of the spacer plates is constituted by about 40 small plates which are obtained from a thin zirconium alloy sheet by blanking and press-forming. The small plates are then assembled and interconnected to constitute the spacer-grid. The assembly of a grid requires about 700 welds to be effected. Furthermore, added elements may be associated with the small plates for the maintenance of the fuel rods in the passages or cells of the grid.
The grids formed in this way result in wide cumulated tolerance ranges.
The structure of a spacer-grid of a fuel assembly for a reactor cooled with light water is therefore extremely complex and requires long and costly operations for its manufacture. Furthermore, the present evolution in the design of fuel assemblies tends toward an increase in the density of the network of fuel rods which generally results in practice in changing from a square lattice network to a triangular lattice network. Lastly, the spacer-grids must also perform functions other than the mere transverse maintenance of the fuel rods, for example functions concerning the distribution and mixture of the cooling water circulating in the assembly.
All these requirements still further complicate the design and manufacture of the spacer-grids.
Fuel elements are known which are intended for nuclear reactors cooled with heavy water and which are constituted by cylindrical rods of fissile or fertile material mounted within a tubular case. French Patent No. 2 028 039, discloses a spacer for such a fuel element, constituted by an element in the form of a machined disc which defines passages for the fuel rods. The zones of contact of the passages with the fuel rods are constituted by cylindrical surfaces whose radius of curvature is a little larger than the radius of curvature of the fuel rods.
Such spacers, which have a radiating symmetry and have a small number of passages for the fuel rods, are in no way comparable to spacer-grids for a fuel assembly of a light water nuclear reactor whose rods are disposed in a dense square or triangular lattice network. For example, an assembly of a pressurized water nuclear reactor contains, in a current configuration, 264 fuel rods disposed in a square lattice network and 25 guide tubes disposed symmetrically in the grid.
Furthermore, the fuel elements for which the spacers described in French Patent No. 2 028 039 are intended do not include guide tubes, are placed in a horizontal position in the nuclear reactor and have a structure which is in no way comparable to that of an assembly for a light water reactor.